Process and device for purifying air

ABSTRACT

A process and device in which air or another gas mixture is contacted with a liquid whereby particulate impurities are washed from the gas and whereby sulfur dioxide and other gaseous pollutants, together with oxygen, are absorbed from the gas into the liquid whereupon said sulfur dioxide and other gaseous pollutants are oxidized to other more soluble chemical forms with the assistance of a catalyst present within the system.

United States Patent 11 1 Coughlin 1 1 Jan. 16, 1973 541 PROCESS ANDDEVICE FOR 2,886,131 5/1959 Conlisk et al ..55/74 x PURIFYING AIR3.328.941 7/1967 Green i .55 243 x 3,405,508 lO/l968 Peters et al.....55/73 [761 Invent: Robert Cwghlm, 902 Seventh 3,291,582 12 1966 Tyson..55 74x Avenue, Bethlehem, 3018 3,416,293 l2/l968 Alexander .55/73 [22}Filed: Oct. 19, 1970 Primary Examiner-John Adee [2]] Appl. No.: 81,877

[57] ABSTRACT [52] U.S. Cl. ..55/73, 55/74, 55/77, A 9

process and device in WhlCh air or another gas m1x- 55/91 55/234 55/24323/4 23/162 ture is contacted with a liquid whereby particulate im-23/l68 23/175 23/288 purities are washed from the gas and whereby sulfur2 dioxide and other gaseous pollutants, together with 1 0 Sean: al 5oxygen, are absorbed from the gas into the liquid I l whereupon saidsulfur dioxide and other gaseous pol- [56] R d lutants are oxidized toother more soluble chemical e erences l 8 forms with the assistance of acatalyst present within UNITED STATES PATENTS the 1 3,598,521 8/1971Allen 255/73 X 10 Claims, 2 Drawing Figures PATENTEDJMI 16 I9733.710.548

INVENTOR.

Robert! Cough/in PROCESS AND DEVICE FOR PURIFYING AIR BACKGROUND OF THEINVENTION 1 Field of Invention This invention relates to a process andto a device for purifying air or other gas mixtures from pollutants bycontacting with a liquid which washes particulate impurities from thegas and which also absorbs from said gas sulfur dioxide and othergaseous pollutants as well as oxygen, whereupon said sulfur dioxide andother gaseous impurities are oxidized to other, more soluble chemicalforms with the assistance of a catalyst present in the system.

2. Description Of Prior Art Processes and devices for purifying air forbreathing in homes, offices, institutions, etc. have relied primarily onfiltration and electrostatic techniques for removing dusts andparticulates. In recent times the importance of removing gaseouspollutants has also become apparent and, at present, activated carbon isused to some extent to remove gaseous pollutants from air. However, insuch cases, when the activated carbon becomes saturated with pollutantsit must be replaced or regenerated. Moreover, the many differentindustrialtype processes for removing S from gases also require aregeneration of the sorbent or other complex operations or steps that donot make for simple, .convenient devices or processes well adapted foruse in homes, offices, institutions, hotels, etc.

Until the present invention, no simple process and device has beenavailable for removing from ambient air both gaseous andparticulate-matter pollutants and simultaneously concentrating saidpollutants into a small volume of liquid that can be easily discardedafter the pollutants have reached a suitable concentration leveltherein. The present invention makes use of both aliquid and a catalystto accomplish this but the present invention differs from prior art inthat gaseous pollutants are absorbed into the liquid before they areoxidized, and in that the oxidation catalyst may be either homogeneous,(i.e., dissolved in the liquid) or heterogeneous (i.e., a solid), and inthat, when the catalyst is activated carbon, it isnot necessary toimpregnate the active carbon with another catalytically activesubstance. These aspects distinguish the present invention from suchprior art as is contained in U.S. Pat. Nos. 3,345,125; 3,318,662;3,473,297; 3,150,923; 3,405,508 and 3,486,852.

' SUMMARY OF THE INVENTION It is an object of this invention to providea means of removing from air, or from another gaseous mixture, thepollutant sulfur dioxide and other gaseous pollutants as well asparticulate-matter, pollutant impurities.

It is another object of this invention to provide a process wherebygaseous and particulate pollutants are removed from air or from anothergas mixture by concentrating said pollutants or their reaction productsin a liquid.

It is a further object of this invention to provide a process whichremoves gaseous pollutants from air or from another gaseous mixture byabsorbing said pollutants, together with oxygen, into a liquid whereuponsaid pollutants are oxidized to forms more soluble in the liquid throughthe assistance of a catalyst present within the system.

Still another object of this invention is to provide a means of treatingair to remove pollutants present in 253 air, whereby said poilutants ortheir reaction products become concentrated in a liquid such as water oran aqueous solution.

Yet another object of this invention is to provide a device adapted forhome, office or institutional use that will purify air by removingpollutants and concentrating them or their reaction products in a liquidsuch as water or in an aqueous solution that can be periodicallydiscarded and replenished.

A further object is to provide a device of the above type in which theliquid can be recycled for contacting the air, because the absorbedgaseous pollutants are converted to different chemical forms which donot interfere, or do not strongly interfere, with the ability of theliquid to absorb additional pollutants from the air.

These and other objects have now herein been attained by a process inwhich air containing sulfur dioxide and particulate-matter pollutants iscounter-currently contacted with water whereby the particulateimpurities enter the water and the sulfur dioxide together with oxygenis absorbed by the water, whereupon the sulfur dioxide is oxidized bythe oxygen with the assistance of a homogeneous catalyst dissolved inthe water or of a heterogeneous catalyst with which the water also comesinto contact.

DESCRIPTION OF THE DRAWINGS Other objects and advantages of theinvention will appear during the course of the following descriptions inconnection with the accompanying drawings which exemplify forms theinvention may assume.

FIG. 1 is a schematic diagram of a device for carrying out the processof this invention by using a packed gasabsorption column in which liquidflows downward over the surface of the packing and gas flows upwardsthrough the interstices between pieces of packing.

FIG. 2 is a schematic diagram of a device for carrying out the processof this invention by using a moving-belt of porous material to bringliquid into contact with the gis sfi iiinl DESCRIPTION OF PREFERREDEMBODIMENTS V The embodiments discussed here are presented herein forpurposes of illustration only and are not intended to be limiting in anymanner.

Referring to FIG. 1, the number 10 designates a cylindrical columnpacked with solid packing material 11, over which liquid flows in adownward direction under the influence of gravity. The packing 11 issupported by screen 12 below which is a vapor space 13 through which theliquid leaving the packing falls into the liquid supply 14 contained inthe cylindrical-reservoir l5 portion of the column. Gas enters the inlet16 of the blower 17 whereby it is propelled into the vapor space 13. Thebafile 23 is provided to prevent liquid from entering blower 17. Liquidis pumped by pump 18 from the reservoir 15 through three-way valve 20 tothe top of the column where it passes through the distributor 19 to thetop of the packing. The three-way valve 20 is provided to allow pumpingmost of the liquid out of the device when necessary; liquid remaining inthe lower portion of the reservoir 15, including any accumulated soliddebris or particulate-matter impurities, can be drained from the bottomof the device through valve 21. The catalyst for this process may be ahomogeneous catalyst dissolved in the liquid; or it may be aheterogeneous catalyst mixed with the packing, or located in the lowerportion of the packed secton above packing support 12, or situatedwithin the reservoir 15 in contact with the liquid 14. Alternatively,the entire packed section of the column may be filled with a solidheterogeneous catalyst which then also serves as a packing 11 to promotegas absorption and removal of impurities from the gas into the water.

The operation of the device of FIG. 1 is as follows: Liquid is pumpedfrom the reservoir 15 by pump 18 through valve to the distributor 19which introduces said liquid to the top of the packing 11. Liquid flowsdownward by gravity over packing l1 and simultaneously contacts gasflowing upward through packing 11. Gas to be purified enters the intake16 of blower 17 whereby it is propelled into space 13 and then upwardsthrough packing support 12 and packing 11; the gas finally leaves thecolumn through the top of the column 22. As the liquid contacts the gasin packed section 11, particulate-matter impurities are washed from thegas into the liquid and gaseous impurities and oxygen are absorbed fromthe gas into the liquid, whereby the gas which leaves the top 22 of thecolumn is depleted in impurities and thus purified. Impurities, such assulfur dioxide thus absorbed into the liquid are oxidized to moresoluble, non-volatile products, such as sulfuric acid, by reacting withthe oxygen also absorbed in the liquid and with the assistance of acatalyst present in the system. Such catalyst may be a homogeneouscatalyst dissolved in the liquid or it may be present in the form of asolid material (heterogeneous catalyst) within the packed section ofcolumn 10 or within the reservoir 15. Particulate-matter impurities arecarried by the liquid into the reservoir 15 where they settle to thebottom. The liquid is recycled from the reservoir 15 to the distributor19 by the pump 18. Upon continuous recycle, the liquid continues able toabsorb gaseous impurities because said gaseous impurities are oxidized,after absorption, to other chemical forms (by the oxygen also absorbedin the liquid) with the assistance of the catalyst present in thesystem. These chemicalreaction products of higher oxidation state do notinterfere markedly with continued absorption and, in this way, theliquid is prevented from attaining equilibrium with gaseous impuritiesin the gas to be purified.

The device shown in FIG. 1 can be filled with liquid by merely pouringthrough the top 22 of the device. After operation, most of the liquid(now containing impurities) can be pumped from the device through valve20 (set in the suitable position); the remainder of the liquid (withimpurities) can be drained by gravity from the device through valve 21.It is clear that if a homogeneous catalyst, rather than a heterogeneouscatalyst, is used it can he added with fresh liquid when the system isfilled.

Referring to the device depicted in FIG. 2, the gas enters the systemthrough inlet 26 of the blower whereby it is propelled through sections27 and 27' of the flexible, porous moving belt 28. The flexible, porousmoving belt28 is wet with liquid which it has picked up from liquidsupply 24 in reservoir 30. After contacting the porous wetted belt 28,the gas passes from the system. The porous wetted belt 28 is stretchedover drums 29 and 29' which are driven by a motor not shown and whichthereby cause the belt to move. The porous, moving belt saturated withliquid from the reservoir 30 brings this liquid into contact with gasesleaving the system, whereby particulate impurities are removed byfiltering action and gaseous impurities and oxygen are absorbed by theliquid transported by the belt 27'. After passing over the upper drum29, the belt 27 encounters gas leaving the blower 25 and, again in thisregion, the belt also filters particulate-matter impurities from the gasand also absorbs oxygen and gaseous impurities therefrom. Finally, thebelt enters the reservoir where it deposits particulate impurities.Gaseous impurities absorbed into the liquid are oxidized, as in the caseof the device of FIG. 1, by oxygen also absorbed in the liquid and withthe assistance of a catalyst present within the system. Said catalystmay be a homogeneous catalyst dissolved in the liquid or it may be asolid heterogeneous catalyst contained within the porous, flexiblemoving belt or within the reservoir 30.

In case the gas to be purified contains a large concentration ofparticulate-matter impurities it is clear that it may be desirable toplace a separate filter for particulates at the gas inlet of either thedevice of FIG. 1 or that of FIG. 2, in order to avoid possible pluggingof the packing in the first case or of the porous belt in the secondcase.

EXAMPLE I Air containing 1 ppm of sulfur dioxide is passed at the rateof about 2,250 ft"/hr through a cylindrical column, about one foot indiameter and about two feet high, packed with z-inch Raschig Rings. Purewater flows downward over the packing in this column at a flow rate ofabout one gallon/minute. The air and water are essentially at roomtemperature. Using standard correlations for mass transfer, one computesthat about 94 percent of the sulfur dioxide is absorbed from the air bythe water (about 7| percent is absorbed if the column is assumed to beabout 1 foot high). Thus, this a simple method for purifying the air ina room since 2,250 ft is about the volume ofa small room. However, thewater leaving the column does not retain its original ability to absorbsulfur dioxide from the air since, after passing once through thecolumn, it exits with a sulfurdioxide concentration of about 0.2 ppm; ifa fixed volume of water is continuously recycled through the column, asin FIG. 1, the amount of sulfur dioxide removed from the air willdecrease continuously as the water approaches a concentrationequilibrium with the air'. However, as the water absorbs sulfur dioxidefrom the air it also absorbs oxygen and it can be computed that the rateof absorption into pure water of oxygen from air is faster than that ofsulfur dioxide from air in which the S0 concentration is l ppm.Therefore, there will be more than sufficient oxygen absorbed into thewater along with the S0 to oxidize the sulfur dioxide to sulfuric acid;however, a catalyst will usually be needed to insure that the rate ofoxidation of sulfur dioxide is fast enough to keep up with the rate ofabsorption of that gas so that the sulfur-dioxide concentration will notbuild up in the water due to the conversion of said S0 to sulfuric acid;said sulfuric acid, even in concentrations up to several percent, willnot materially interfere with the further absorption of sulfur dioxidefrom the air by the water when the water is recycled.

Based on studies of the oxidation of sulfite ion carried out by Fuller[Journal of the American Chemical Society 63, p. 1644 (1941)] and Yagi[Chemical Engineering Science 17, p. 411 (1962)] cupric ion and cobalticion dissolved in water are good homogeneous catalysts for the oxidationof S0 in water containing dissolved oxygen. Using the results of Yagiand those of Fuller, and assuming the system contains about one gallonof water, one computes that about 0.01 gm mole per liter of C030 or ofCuSO. should be a sufficient concentration of homogeneous catalyst toinsure that the water recycled to the top of the column in this examplecontains little of the originally absorbed S0 in a form that willinterfere with the absorption of additional S0 from the air, a largeproportion of the absorbed S0 having been homogeneously oxidized to S0and sulfuric acid by the dissolved oxygen with the assistance of thehomogeneous catalyst. Based on the claims of Pauling (U.S. Pat. No.3,318,662) suitable homogeneous catalysts for the present example couldbe soluble salts of Mn,Cu,Ti,Fe,Zn,Ni,Co,Cr,V,Mo and Sn. Some suitablepacking materials for the absorption column of this example wouldinclude ceramic rings, saddles or spirals, various shapes of plastictower packings or standard metallic tower packings, preferably of acorrosion resistant metal like a stainless steel.

EXAMPLE ll This example makes use of the same system as Example I excepta heterogeneous catalyst, such as activated carbon or oxides ofMn,Cu,Ti,Fe,Zn,Ni,Co,Cr,V,Mo, or Sn, is included in the system insteadof, or together with, the homogeneous catalyst of Example I. Saidheterogeneous catalyst may be placed in contact with the liquid in thereservoir below the packing; or it may be in the fonn of fine particlescarried by the water in slurry form; or it may be mixed with thepacking; or it may actually serve as the packing for the gas absorptioncolumn.

Ovchinnikova and Davtyan [Zhur. Fiz. Khim. 30,

1735 (1956)] studied the use of activated carbon as a heterogeneouscatalyst for the oxidation of $0 to sulfuric acid in a system containingwater and oxygen; based on their results, one estimates that about 22 lbof granular activated carbon [bulk density 22 lb/ft"10 X40 mesh particlesize] should possess sufficient catalytieactivity to produce thereaction rate of sulfur dioxide oxidation required in Example I. Thisamount of granular carbon would about fill a cylindrical column about 1ft in diameter and about 1 foot high. Said activated carbon catalystcould conveniently be mixed with an inert packing material to fill thegas absorption part of the device of FIG. 1; or the entire packing couldbe of activated carbon of suitable particle size which would supply morethan adequate catalytic activity; or the activated carbon could besuspended as fine particles in the water to form a slurry therewith; orthe carbon could be located within the reservoir in contact with thewater therein.

In each of the examples above the water can be recycled continuouslyuntil the concentration of sulfuric acid builds up to the point where itbegins to severly interfere with the absorption and oxidation of $0 Forexample, if the H SO is allowed to build up in the water in the devicesof Example I or Example ll until a concentration of 10 percent isachieved, this represents about 20 days of operation; clearly, it wouldnot be inconvenient to change the water every 20 days or even morefrequently. Moreover, if the air to be treated is very dry evaporationof water may take place, in which case it would be necessary to addmake-up water to the system as needed. Finally it should be pointed outthat although the discussion here has been concerned largely with theabsorption and oxidation of the gaseous pollutant sulfur dioxide US.Pat. No. 2,992,895 suggest that the lower oxides of nitrogen will beabsorbed and oxidized in a similar fashion, at least when anactivecarbon heterogeneous catalyst is used in the system.

From the foregoing descriptions of preferred forms of the invention, itis evident that it provides a simple and effective means of purifyingair, removing not only particulate-matter pollutants but importantgaseous pollutants as well. Said pollutants become concentrated within arelatively small volume of liquid which can conveniently be discardedfrom time to time.

Having fully described the invention, it will be apparent to one havingordinary skill in the art that it is not limited to the preciseconfigurations illustrated and that many modifications and changes canbe made without departing from the spirit and scope thereof.

What is claimed and intended to be covered by Letters Patent is:

1. A process of removing from a gas stream, which contains an excess ofoxygen, an oxide of a multivalent non-metal selected from the groupwhich consists of sulfur and nitrogen, comprising,

a. contacting said gas with an aqueous liquid in the presence of acatalyst for the chemical reaction of said oxide to a more soluble form;

b. recirculating said aqueous liquid into contact with said gas stream;

0. recovering or accumulating in a body of said aqueous liquid a productwhich contains an aqueous solution of said more soluble form of theoxide of aforesaid multivalent non-metals.

2. The process of claim 1 wherein the gas is air and the temperature ofoperation is about room temperature.

3. The process of claim 2 wherein particulate matter is also filteredfrom the gas.

4. The process of claim 2 wherein the catalyst is selected from thegroup which consists of compounds ofCu, C0, Mn, Fe, Ni, Cr, and V.

5. The process of claim 4 wherein particulate matter is also filteredfrom the gas and wherein the concentration of said catalyst exceedsabout 30 parts by weight per one million parts by weight of said water.

6. The process of claim 3 wherein the catalyst is a carbonaceousmaterial.

7. An apparatus for removing an oxide of sulfur or nitrogen from air,wherein oxygen and said oxide are absorbed into an aqueous liquid,wherein said oxide is oxidized to more soluble form with the assistanceof a catalyst, comprising means forming areservoir, a body of liquid insaid reservoir, catalyst means in contact with said liquid, first andsecond drum means, said first drum means being positioned at leastpartly below the surface of said body of water and said second drummeans positioned above said body, a flexible, porous movable filter beltpositioned on said drum means,

means is contained within said filter belt.

9. The apparatus of claim 7 wherein said catalyst means is containedwithin said body of liquid.

10. The apparatus of claim 7 wherein said catalyst means is acarbonaceous material.

t l l

2. The process of claim 1 wherein the gas is air and the temperature ofoperation is about room temperature.
 3. The process of claim 2 whereinparticulate matter is also filtered from the gas.
 4. The process ofclaim 2 wherein the catalyst is selected from the group which consistsof compounds of Cu, Co, Mn, Fe, Ni, Cr, and V.
 5. The process of claim 4wherein particulate matter is also filtered from the gas and wherein theconcentration of said catalyst exceeds about 30 parts by weight per onemillion parts by weight of said water.
 6. The process of claim 3 whereinthe catalyst is a carbonaceous material.
 7. An apparatus for removing anoxide of sulfur or nitrogen from air, wherein oxygen and said oxide areabsorbed into an aqueous liquid, wherein said oxide is oxidized to moresoluble form with the assistance of a catalyst, comprising means forminga reservoir, a body of liquid in said reservoir, catalyst means incontact with said liquid, first and second drum means, said first drummeans being positioned at least partly below the surface of said body ofwater and said second drum means positioned above said body, a flexible,porous movable filter belt positioned on said drum means, blower meanspositioned to force said air through said filter belt and means to movesaid belt, after contacting said gas, into and out of said reservoirwhereby particulate impurities are removed by filtering action andgaseous impurities and oxygen are absorbed by the liquid transported bysaid belt.
 8. The apparatus of claim 7 wherein said catalyst means iscontained within said filter belt.
 9. The apparatus of claim 7 whereinsaid catalyst means is contained within said body of liquid.
 10. Theapparatus of claim 7 wherein said catalyst means is a carbonaceousmaterial.